Two measurements, taken on two separate dates in late 2025, put a number on how strange this thing is: methanol-to-hydrogen-cyanide ratios of roughly 70 and 120.
That’s not a marginal result. Those values land 3I/ATLAS among the most methanol-rich comets ever studied, and the comparison set here includes basically every comet astronomers have ever pointed a radio dish at in our own solar system.
The observations came from the Atacama Large Millimeter/submillimeter Array in Chile, specifically its Atacama Compact Array. The U.S. National Science Foundation National Radio Astronomy Observatory is a partner in ALMA.
What a fingerprint from another star actually looks like
"Observing 3I/ATLAS is like taking a fingerprint from another solar system," said Nathan Roth, lead author on this research and a professor with American University. "The details reveal what it’s made of, and it’s bursting with methanol in a way we just don’t usually see in comets in our own solar system."
The method is straightforward, even if the execution isn’t. As the comet moved closer to the Sun through late 2025, sunlight heated its icy surface. Gas and dust escaped, forming a bright cloud around the core, the coma.
Read the molecules in that coma and you’re reading the comet’s composition. And because 3I/ATLAS came from outside our solar system, you’re reading the composition of something assembled somewhere else entirely. No spacecraft required.
The team went after the faint submillimeter signatures of two molecules. Methanol, which is a type of alcohol, and hydrogen cyanide, a nitrogen-bearing organic molecule that turns up routinely in comets.
The ice didn’t form the way ours did
A ratio that lopsided points somewhere specific. The ice inside 3I/ATLAS either formed under conditions very different from those most solar system comets experienced, or it was exposed to such conditions later.
This isn’t the comet’s first oddity either. Earlier observations with the James Webb Space Telescope found a coma dominated by carbon dioxide while 3I/ATLAS was still far from the Sun. Abundant methanol now joins that list.
Two molecules, two very different exits
Here’s the part I find more interesting than the ratio itself. ALMA’s imaging resolution let astronomers track not just what molecules are present but where they’re coming from, and the two behave nothing alike.
Hydrogen cyanide comes mostly from the nucleus, the comet’s central body. Textbook behavior, matching what solar system comets do.
Methanol doesn’t play along. It appears to come from the nucleus and from ice particles floating within the coma itself.
Mini-comets inside the comet
Those icy grains function as miniature comets. As 3I/ATLAS gets closer to the Sun and temperatures climb, the ice inside the grains turns to gas and dumps more methanol into the coma.
Astronomers have watched this happen before in comets from our own solar system, so the mechanism isn’t new. What’s new is tracing the detailed physics of that outgassing in an object born in interstellar space. First time anyone has done it.
3I/ATLAS is only the third confirmed interstellar object observed entering our solar system, after 1I/’Oumuamua and 2I/Borisov. Both of those earlier visitors turned up unusual characteristics of their own, which is starting to look less like coincidence and more like a pattern worth explaining.
Every new interstellar object is another chance to hold our solar system up against a planetary system somewhere else in the galaxy. Three data points isn’t a sample. But if you want to know whether the chemistry that built our comets is typical or a fluke, the methanol numbers on 3I/ATLAS are the kind of measurement that starts answering it.
